The electronic configuration of oxygen is 1s² 2s² 2p_x² 2p_y¹2p_z¹ i.e. it has two half-filled orbital and there is no d-orbital available for excitation of electron. Further, it is the most electronegative element of its family. Hence it shows oxidation state -2 only. Other elements like sulphur have d-orbital available for excitation, thereby giving four and six half-filled orbitals; moreover they can combine with more electronegative elements. Hence they shows oxidation states of +2,+4 and +6 also.